1. Field of the Invention
The invention relates to a device for displaying television pictures by means of a display tube in the neck of which an electron gun system is present for emitting at least one electron beam towards a display screen and having an electromagnetic deflection unit which is disposed around a portion of the display tube and which comprises a first deflection coil for producing, when energized, a frame deflection field and a second deflection coil for producing, when energized, a line deflection field, said second deflection coil being located coaxially within the first deflection coil, each deflection coil having two diametrically oppositely located coil units the first deflection coil (the frame deflection coil) having a winding distribution for producing, upon energization, a dipole field in combination with a positive sixpole field on the side of the gun system, producing a dipole field in combination with a negative sixpole field in the central area, and producing a dipole field in combination with a positive sixpole field on the side facing the display screen.
2. Description of the Prior Art
In monochrome display tubes the electron gun system is designed to produce one electron beam. In colour display tubes the electron gun system is designed to produce three electron beams.
For some time now colour display tubes have been used in which three electron beams which are spatially separated from each other are located in one plane. Such a display tube is known as an in-line colour display tube. In the in-line colour display tube it is endeavoured to use a deflection unit having deflection coils which give such an inhomogeneous field distribution that the beams of the electron guns coincide over the whole screen when being deflected. For that purpose in particular the line deflection field (to be generated by the second deflection coil) on the gun side of the deflection yoke must be barrel-shaped and towards the screen side it must be pincushion-shaped, and, conversely, the frame deflection field (to be generated by the first deflection coil) must be pincushion-shaped on the gun side and it must be barrel-shaped more towards the screen side.
The extent of pincushion shape and barrel shape is such that upon deflection the convergence errors of the electron beams emitted by the electron guns are corrected as a result of which pictures of satisfactory convergence properties can be produced on the screen of the display tube. Display tube-deflection yoke combinations of this type are said to be self-converging.
Although the convergence is ensured in this manner (for that purpose, in practice, the deflection coils must often be combined with auxiliary means for intensifying the pincushion shape and barrel shape of the deflection field, which auxiliary means are, for example, plates of soft-magnetic, metallic material placed in the deflection fields), a certain raster distortion (east-west raster distortion) often proves to occur still on the vertical sides of the display screen which has to be corrected.
Such a design of the first deflection coil (that is the coil for deflecting the electron beams in the vertical direction or frame deflection coil) that upon energisation it produces, on its side facing the gun system, a dipole field in combination with a positive sixpole field (resulting in a pincushion shaped deflection field), produces a dipole field in its central area in combination with a negative sixpole field (resulting in a barrel shaped deflection field), and produces a dipole field on its side facing the display screen in combination with a positive sixpole field (resulting in a pincushion shaped deflection field), may lead to a TV display device which satisfies the requirements imposed as regards self-convergence and raster distortion. However, with the winding methods used nowadays it is very difficult, if not impossible, to produce as a production item a frame deflection coil which has such a strongly varying winding distribution as is necessary to realise the above-described deflection field design. This applies in particular with reference to the winding distribution on the screen side. In order to make the pincushion shape of the frame deflection field on the screen side so pronounced as is necessary for achieving a minimum east-west raster distortion, it is required to make the sixpole component sufficiently positive. The sixpole component becomes more positive as the window aperture, which in saddle coils is defined as the angular aperture between the two axially extending winding segments, becomes larger. However, the strength of the sixpole cannot be made unrestrictedly large. With an angular aperture of 180.degree. the maximum strength is reached. When the angular aperture is made larger than 180.degree., for which purpose the coil units of the saddle coils would have to overlap each other, the strength of the sixpole component decreases. Moreover, the more pronounced the pincushion shape is made on the screen side, the more pronounced must be the barrel shape in the central area to keep the astigmatism error to a minimum and this means that the pincushion shape on the gun side must be proportionally more pronounced to keep the coma error to a minimum. This means that it becomes more and more difficult to manufacture the frame deflection coil. So for practical reasons it would be desirable to have a not too pronounced pincushion shape of the frame deflection field on the screen side. The invention provides a solution to this problem. For that purpose, the device of the kind described in the opening paragraph is characterized in that the first and the second deflection coils are positioned in the axial direction relative to each other in such manner that upon energisation the magnetic field generated by the first deflection coil extends so much farther towards the display screen than the magnetic field generated by the second deflection coil that the device shows a minimum east-west raster distortion.
Since the frame deflection field extends farther towards the screen than the line deflection field, not too strongly pronounced a pincushion shape of the frame deflection field in the screen side area will suffice while nevertheless the requirement of a minimum east-west raster error is satisfied and it is easier to wind the frame deflection coil in such manner that the requirements of minimum convergence errors (astigmatism and coma errors) are fulfilled.
The deflection units for television display devices having both a line deflection coil and a frame deflection coil of the saddle-type have so far been constructed so that the deflection field generated by the line deflection coil on the screen side extended farther than the deflection field generated by the frame deflection coil. The invention on the contrary uses the recognition that by causing the frame deflection field to extend farther to the screen side than the line deflection field, east-west raster errors can be minimised.